Regarding SMTP email servers, what is an MUA?

Regarding SMTP email servers, what is an MUA?

• Mail User Agent
• Mail Utility Activator
• Multiple User Activator
• Multiple User Agent 

EXPLANATION

Retrieving mail is done using a software program called an MUA (Mail User Agent). When the MUA is a program installed on the user's system, it is called an email client (such as Mozilla Thunderbird, Microsoft Outlook, Eudora Mail, Incredimail or Lotus Notes).